Solute-phase aptamer sensing with aptamer isolation

ABSTRACT

A sensing device for sensing at least one analyte in a sample solution is provided. The device 100 includes a sensor fluid 18 including an aptamer, and the aptamer is configured to measure an analyte included in a sample fluid 14, the aptamer configured to freely diffuse in the sensor fluid. The device also includes at least one isolation element (e.g., membrane 136) retaining the aptamer in the sensor fluid. A method of fabrication of a sensing device for sensing at least one analyte in a sample solution is provided. The method includes defining a volume 130 configured to house a sample fluid with at least one analyte, measuring an analyte included in a sensor fluid that is at least in part measured by an aptamer freely diffusing in the sensor fluid, and retaining the aptamer in the sensor fluid with at least one isolation element. A method of sensing an analyte in a sample fluid is also provided. The method includes bringing an analyte included in a sample fluid into contact with an aptamer included in a sensor fluid, resulting in a change in the electron transfer between a redox tag and an electrode, and measuring the change in electron transfer between the redox tag and the electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/082,834, filed on Sep. 24, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/082,999, filed on Sep. 24, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/083,029, filed on Sep. 24, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/085,484, filed on Sep. 30, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/122,071, filed on Dec. 7, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/122,076, filed on Dec. 7, 2020; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/136,262, filed on Jan. 12, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,667, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,677, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,712, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,856, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,865, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,894, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,944, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,953, filed on Feb. 18, 2021; claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/150,986, filed on Feb. 18, 2021; and claims the benefit of the filing date of U.S. Patent Application Ser. No. 63/197,674, filed on Jun. 7, 2021, the disclosures of each of which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

Electrochemical aptamer sensors can identify the presence and/or concentration of an analyte of interest via the use of an aptamer sequence that specifically binds to the analyte of interest. These sensors include aptamers attached to an electrode, wherein each of the aptamers has a redox active molecule (redox couple) attached thereto. The redox couple can transfer electrical charge to or from the electrode. When an analyte binds to the aptamer, the aptamer changes shape, bringing the redox couple closer to or further from, on average, the electrode. This results in a measurable change in electrical current that can be translated to a measure of concentration of the analyte.

A major unresolved challenge for aptamer sensors (particularly those where the aptamers are bonded to the working electrode) is the lifetime of the sensors, especially for applications where continuous operation is required (“continuous” referring to multiple measurements over time by the same device). To date, it has been difficult to provide electrochemical aptamer sensors with a lifetime that allows continuous sensing to take place over an extended period of time. Furthermore, for aptamer sensors where the aptamer is bonded to the electrode, the flexibility of design is limited and often the sensitivity of the aptamer therefore suffers as a consequence.

What is needed are devices and methods to simultaneously resolve challenges for aptamer sensors.

SUMMARY OF THE INVENTION

Certain exemplary aspects of the invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be explicitly set forth below.

Many of the drawbacks and limitations stated above can be resolved by creating novel and advanced interplays of chemicals, materials, sensors, electronics, microfluidics, algorithms, computing, software, systems, and other features or designs, in a manner that affordably, effectively, conveniently, intelligently, or reliably brings sensing technology into proximity with biofluid and analytes.

Various aspects of the disclosed invention are directed to a sensing device, a method of fabricating a sensing device, and a method of sensing an analyte in a sample fluid. As mentioned, one aspect provides a sensing device for sensing an analyte in a sample fluid. The sensing device includes a sensor fluid including a plurality of aptamers, at least one or more of the aptamers being configured to sense an analyte included in a sample fluid (such as by binding thereto). In various embodiments, the aptamer is freely diffusing in the sensor fluid. The sensing device further includes at least one isolation element that retains the plurality of aptamers in the sensor fluid.

In another embodiment, a method of fabricating a sensing device for sensing an analyte in a sample fluid is provided. The method includes defining a first volume configured to house a sample fluid with at least one analyte. The method further includes defining a second volume configured to house a sensor fluid, the sensor fluid including an analyte that is at least in part measured by an aptamer freely diffusing in the sensor fluid. The method further includes retaining the aptamer in the sensor fluid with at least one isolation element.

In another embodiment, a method of sensing an analyte in a sample solution is provided. The method includes bringing an analyte included in a sample fluid into contact with an aptamer included in a sensor fluid while the aptamer is freely diffusing in the sensor fluid. The contact of the aptamer with the analyte results in a change in the electron transfer between a redox tag and an electrode. The method further includes measuring the change in electron transfer between the redox tag and the electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the disclosed invention will be further appreciated in light of the following detailed descriptions and drawings in which:

FIG. 1A is a cross-sectional view of a device in accordance with principles of the disclosed invention.

FIG. 1B is a cross-sectional view of an alternate embodiment of a device in accordance with principles of the disclosed invention.

FIG. 2A is a schematic showing a prior art portion of an aptamer sensor device having a passivating layer and an aptamer attached to an electrode.

FIG. 2B is a schematic showing the aptamer and passivating layer portions of the aptamer sensor device of FIG. 2A degrading over time.

FIG. 3 is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 4 is a graph showing the effect of a membrane in an aptamer sensor device on percentage of solute retention versus molecular weight of the solute.

FIG. 5A is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 5B is a schematic showing an alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 5C is a schematic showing yet another alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 5D is a schematic showing yet another alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 5E is a schematic showing yet another alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 6A is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 6B is a schematic showing an alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 6C is a schematic showing yet another alternate embodiment of an aptamer with attached redox tag that can be used with devices in accordance with principles of the disclosed invention.

FIG. 7 is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 8 is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 9 is a schematic of yet another embodiment of a device in accordance with principles of the present invention.

FIG. 10A is a graph showing raw chronoamperometric scans of current versus time for cortisol in an exemplary device.

FIG. 10B is a graph showing normalized current gain for three sensors versus concentration of cortisol.

DEFINITIONS

As used herein, “continuous sensing” with a “continuous sensor” means a sensor that changes in response to changing concentration of at least one solute in a solution such as an analyte. Similarly, as used herein, “continuous monitoring” means the capability of a device to provide multiple measurements of an analyte over time.

As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, pH, size, concentration or percentage is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified amount, as such variations are appropriate to perform the disclosed method.

As used herein, the term “electrode” means any material that is electrically conductive such as gold, platinum, nickel, silicon, conductive liquid infused materials such as ionic liquids, PEDOT:PSS, conductive oxides, carbon, boron-doped diamond, nanotubes or nanowire meshes, or other suitable electrically conducting materials.

As used herein, the term “blocking layer” or “passivating layer” means a homogeneous or heterogeneous layer of molecules on an electrode which alter the electrochemical behavior of the electrode. Examples include a monolayer of mercaptohexanol on a gold electrode or as another example natural small-molecule solutes in serum that form a layer on a carbon electrode.

As used herein, the term “aptamer” means a molecule that undergoes a conformation or binding change as an analyte binds to the molecule, and which satisfies the general operating principles of the sensing method as described herein. Such molecules are, e.g., natural or modified DNA, RNA, or XNA oligonucleotide sequences, spiegelmers, peptide aptamers, and affimers. Modifications may include substituting unnatural nucleic acid bases for natural bases within the aptamer sequence, replacing natural sequences with unnatural sequences, or other suitable modifications that improve sensor function, but which behave analogous to traditional aptamers. Two or more aptamers bound together can also be referred to as an aptamer (i.e. not separated in solution). Aptamers can have molecular weights of at least 1 kDa, 10 kDa, or 100 kDa.

As used herein, the term “redox tag” or “redox molecule” means any species such as small or large molecules with a redox active portion that when brought adjacent to an electrode can reversibly transfer at least one electron with the electrode. Redox tag or molecule examples include methylene blue, ferrocene, quinones, or other suitable species that satisfy the definition of a redox tag or molecule. In some cases, a redox tag or molecule is referred to as a redox mediator. Redox tags or molecules may also exchange electrons with other redox tags or molecules.

As used herein, the term “change in electron transfer” means a redox tag whose electron transfer with an electrode has changed in a measurable manner. This change in electron transfer can, for example, originate from availability for electron transfer, distance from an electrode, diffusion rate to or from an electrode, a shift or increase or decrease in electrochemical activity of the redox tag, or any other embodiment as taught herein that results in a measurable change in electron transfer between the redox tag and the electrode.

As used herein, the term “fluorescent tag” and “fluorescent quencher’ means molecules which are like those used in molecular beacon laboratory assays. Examples of fluorescent tags include 6-FAM (carboxylflourescien), JOE, TET, HEX, and examples of quenchers include black-hole quenchers, DABCYL.

As used herein, the term “signaling aptamer” means an aptamer that is tagged with a redox active molecule or tag and/or contains a redox active portion itself and which provides a change in electrochemical signal when it is released from an anchor aptamer.

As used herein, the term “anchor aptamer” means an aptamer that that can bind to a signaling aptamer, and when bound to the signaling aptamer changes at least one property of the bound vs. unbound signaling aptamer such as molecular weight, diffusion coefficient, charge state, being floating in solution vs. being immobilized, or some other property which achieves the stated effect for the signaling aptamer. The binding of the anchor aptamer with the signaling aptamer is dependent on concentration of the analyte to be measured.

As used herein, the term “folded aptamer” means an aptamer that along its length associates with itself in one or more locations creating a three-dimensional structure for the aptamer that is distinct from an “unfolded aptamer” that is a freely floating and oscillating strand of aptamer. Aptamers can also be partially folded or partially unfolded in structure or in time spent in the folded vs. unfolded states. Multiple folding configurations are also possible.

As used herein, the term “analyte” means any solute in a solution or fluid which can be measured using a sensor. Analytes can be small molecules, proteins, peptides, electrolytes, acids, bases, antibodies, molecules with small molecules bound to them, DNA, RNA, drugs, chemicals, pollutants, or other solutes in a solution or fluid.

As used herein, the term “membrane” means a polymer film, plug of hydrogel, liquid-infused film, tiny pore, or other suitable material which is permiselective to transport of a solute through the membrane by solute parameters such as size, charge state, hydrophobicity, physical structure, or other solute parameters than can enable permiselectivity. For example, a dialysis membrane is permselective by passing small solutes but not large solutes such as proteins. Membranes as understood herein need not be multiporous, for example a nanotube or nanopore can act as a permiselective filter and is therefore considered part of a membrane as understood for the present invention.

As used herein, the term “sample fluid” means any solution or fluid that contains at least one analyte to be measured.

As used herein, the term “sensor fluid” means a solution or fluid that differs from a sample solution by at least one property, and through which the sensor solution and the sample solution are therefore separated but are in fluidic connection through at least one pathway such as a membrane. The sensor solution comprises at least one aptamer as a solute.

As used herein, the term “reservoir fluid” means a solution or fluid that differs from a sample solution by at least one property, and through which the sensor solution and the reservoir solution are in fluidic connection through at least one pathway such as a membrane or a pin-hole connection. A reservoir fluid may have multiple function in a device, for example, by introducing a solute continuously or as needed by diffusion equilibrium into the sensor fluid, or for example removing unwanted solutes from a sensor fluid and acting as a “waste removal element”.

As used herein, a “device” comprises at least one sensor based on at least one aptamer, at least one sensor solution, and at least one sample solution. Devices can sense multiple samples and be in multiple configurations such as a device to measure a pin-prick of blood, or a microneedle or in-dwelling sensor needle to measure interstitial fluid, or a device to measure saliva, tears, sweat, or urine sensor, or a device to measure water pollutants or food processing solutes, or other devices which measure at least one analyte found in a sample solution.

DETAILED DESCRIPTION OF THE INVENTION

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

Certain embodiments of the disclosed invention show sensors as simple individual elements. It is understood that many sensors require two or more electrodes, reference electrodes, or additional supporting technology or features which are not captured in the description herein. Sensors are preferably electrical in nature, but may also include optical such as a LED or laser excitation source and a photodetector, chemical, mechanical, or other known biosensing mechanisms. Sensors can be in duplicate, triplicate, or more, to provide improved data and readings. Sensors may provide continuous or discrete data and/or readings. Certain embodiments of the disclosed invention show sub-components of what would be sensing devices with more sub-components needed for use of the device in various applications, which are known (e.g., a battery, antenna, adhesive), and for purposes of brevity and focus on inventive aspects, such components may not be explicitly shown in the diagrams or described in the embodiments of the disclosed invention. All ranges of parameters disclosed herein include the endpoints of the ranges.

With reference to FIGS. 1A and 1B, exemplary embodiments of devices in accordance with principles of the disclosed invention are shown. Referring first to FIG. 1A, a device 100 is shown as being placed partially in-vivo into the skin 12 of a subject. Skin 12 includes the epidermis 12 a, the dermis 12 b, and the subcutaneous or hypodermis 12 c. The device 100 includes a feature 112 that allows for access to sample fluids from the subject. Such sample fluids may include interstitial fluid (from the dermis 12 b) and/or blood (from a capillary 12 d). In the embodiment shown in FIG. 1A, the feature 112 includes a plurality of microneedles (which may be formed of metal, polymer, semiconductor, glass, or other suitable material). Each of the microneedles 112 projects from a first substrate 108. And each microneedle 112 may include a hollow lumen 132. The device 100 also includes a second substrate 110 (which may be a material such as polymer or glass) having an electrode 150 adjacent thereto. An optional passivating layer 120 may be adjacent to electrode 150, such that electrode 150 is positioned between passivating layer 120 and second substrate 110. Passivating layer 120 includes a compound such as mercaptohexanol or may comprise natural solutes that have diffused into the device 100 from the dermis 12 b.

As can be seen in FIG. 1A, a defined volume 130 is present between first substrate 108 and passivating layer 120. It will be recognized by those of ordinary skill in the art that defined volume 130 does not necessarily have to be defined by first substrate 108 and passivating layer 120—and in embodiments where passivating layer is absent, volume 130 may be defined by first substrate 108 and electrode 150; or, alternatively, may be defined by first substrate 108 and second substrate 110. A sensor fluid 18 may be present within volume 130 (as shown in FIG. 1A). Further, as can be seen in the embodiment of FIG. 1A, at least one membrane 136 is present between first substrate 108 and passivating layer 120, and is positioned adjacent first substrate 108. The at least one membrane 136 may be of various materials or substances—such as a dialysis membrane or hydrogel, for example. In the particular embodiment shown in FIG. 1A, portions of the membrane 136 overlie the boundary between volume 130 and lumens 132 of each microneedle 112. Due to this positioning of membrane 136, volume 130 includes sensor fluid 18, and lumens 132 include sample fluid 14—such as interstitial fluid from dermis 12 b or blood from capillary 12 d. Together the total volume provided by volume 130 and lumens 132 can be a microfluidic component such as channels, a hydrogel, or other suitable material. A diffusion or other fluidic pathway exists from the sample fluid 14, such as interstitial fluid or blood, into volumes 132, 130.

Another embodiment of a device 100 is shown in FIG. 1B. This embodiment also includes first and second substrates 108, 110, microneedles 112 having lumens 132, electrode 150, passivating layer 120, defined volume 130 and at least one membrane 136. As can be seen from FIG. 1B, in this embodiment, electrode 150 and passivating layer 120 are recessed in second substrate 110 (as opposed to the configuration shown in FIG. 1A). Thus, volume 130 is defined by first substrate 108 and combination of second substrate 110 and passivating layer 120. Further, the embodiment as shown in FIG. 1B includes a plurality of membranes 136, with each membrane 136 positioned in a distal end of each lumen 132 of each microneedle 112. Due to this positioning of membranes 136, both volume 130 and lumens 132 include a sensor fluid 18 (and sample fluid is present in, and obtainable from, dermis 12 b and capillary 12 d, for example).

Alternative arrangements and materials to those discussed above with respect to FIGS. 1A and 1B are possible, such as using a single needle or hydrogel polymer microneedles. In addition, one or more of the features of device 100 or the entire device 100 could be implanted into the body and perform similarly as described herein. Furthermore, a device 100 could be fully outside the body, if for example sampling a fluid such as sweat or tears.

Turning now to FIGS. 2A and 2B, where like numerals refer to like features, a portion of a prior art device 200 is shown. Referring to FIG. 2A, an aptamer sensor includes a passivating or blocking layer 248 (including a compound such as mercaptohexanol) attached to an electrode 250 (made from a material such as gold), and having at least one aptamer 270 that is attached to the electrode 250, such as by being thiol-bonded to the electrode 250. The aptamer 270 has at least one redox tag or molecule 240, such as methylene blue, associated therewith. The device 200 is shown as being positioned in a sample fluid 14, such as blood or interstitial fluid (for example). This prior art device 200 may have an analyte (not shown) that binds with the aptamer 270, thereby changing the availability of the redox tag 240 to the electrode 250, such as by bringing it closer to, or further from, the electrode 250. Conventional aptamer sensors can be limited in performance because an aptamer that is bound to an electrode often has a weaker binding affinity to an analyte than an aptamer that is free in solution. In addition, as shown in FIG. 2B, the sensors can degrade as the aptamer 270 and/or blocking layer 248 degrades over time (e.g., chemical degradation, or detaching from the electrode 250). Also, because such prior art devices 200 have relied on exogenous molecules (e.g., mercaptohexanol) for passivation, the passivation layer 248 can also become thicker with fouling from solutes (such as albumin) in the sample fluid 14.

Thus, and with reference now to FIG. 3 , where like numerals refer to like features, an embodiment of the disclosed invention that improves on the prior art devices and reduces or eliminates drawbacks with such devices is shown. To that end, FIG. 3 shows a device 300 (or at least a portion thereof) that includes an electrode 350 and at least one membrane 336 which separates a sample fluid 14 from a sensor fluid 18. The sensor fluid 18 contains a plurality of aptamers 370 having redox tags 340. The electrode 350 may include a passivating layer 348. The passivating layer 348 may comprise one or more endogeneous solutes 16 from the sample fluid 14 itself (or, as initially prepared, the passivating layer 348 may be prepared from molecules that are known to be endogenous to the sample fluid to be tested). Examples of such endogenous molecules 16 include small molecules such as amino acids, hormones, metabolites, or peptides. (Thus, the device 300 shown in FIG. 3 differs from that described above in prior art FIGS. 2A and 2B in that the prior art device described above included an aptamer and an exogenous molecule, such as mercaptohexanol. Similarly, electrode 350 could also contain a passivation layer 348 comprised, at least in part, by an exogenous molecule such as hexanethiol or mercaptohexanol. But, even in that case, the passivation layer could detach from the electrode 350 and be in need of replacement.) By including endogeneous molecules 16 in the passivation layer 348, longer lifetime of the device 300 is achieved because endogenous molecules 16 can leave the electrode 350 as shown by arrow 392 and another endogenous molecule 16 can replace that now-missing molecule as shown by arrow 390. Thus, in a sense, the very molecules in the sample fluid 14 can be used to “repair” the passivation layer as it degrades, thereby extending the life of the device. (As mentioned above, these endogenous molecules can originate from the sample fluid itself, be already present as a deliberate component of the sensor fluid, or could be a mix of the two.) As a non-limiting example, membrane 336 is able to pass in small solutes (e.g., <1 kDa)—for example, an analyte such as cortisol—and passivating solutes 16, such as amino-acids and peptides, but retains the aptamer 370 (with redox tag 340) which is often >10 kDa in molecular weight. If the aptamer 370 with redox tag 340 were not retained by the membrane 336, then aptamer 370 with redox tag 340 could be lost into the body and no longer able be able to provide a measurement of the analyte.

An example of the analysis of the use of a membrane to pass small solutes (small target analyte) while retaining aptamers within device is shown with reference to FIG. 4 , which shows an illustrative plot of solute retention for a membrane such as membrane 336. This is an example only, and shows that if measuring a small analyte such as cortisol (<400 Da) and using a large aptamer (>10 kDa or even >50 kDa) a membrane could be highly permeable to the analyte and poorly permeable to the aptamer. Thus, for example, in various embodiments, membranes of the present invention may have molecular weight cutoffs (i.e., the molecular weight above which a molecule will not easily pass through the membrane) that are at least one of <300 Da, <1000 Da, <3 kDa, <10 kDa, <30 kDa, <100 kDa, <300 kDa. Larger molecular weight cut-off membranes will require larger sized aptamers to prevent the aptamers from potentially escaping the device.

Several additional embodiments will be discussed below. In these additional embodiments, an increase in availability of the redox tag to the electrode can occur as a result of aptamer binding analyte, or, alternatively, without aptamer binding to an analyte. And even though each of the embodiments discussed below (and their respective figures) may show one specific example, the other the embodiments of the invention are not so limited (e.g., the various aptamer/redox tag types can be used across the various embodiments of devices disclosed herein, and vice versa.

Turning now to FIGS. 5A-5E, where like numerals refer to like features: FIG. 5A shows a portion of a device 500 including a substrate 510, a sensor fluid 18, a plurality of aptamers 570 with redox tags 540 free in the sensor fluid 18, a passivation layer 548 of endogenous solutes 16, and an electrode adjacent the substrate 510. Though not part of the device, analytes 19 are also depicted as present in the sensor fluid of the device. The schematic shown in FIG. 5A also depicts an electron transfer event that occurs between a redox tag 540 and the electrode 550. This is shown generally at reference numeral 598, and is a non-limiting example depicting that electron transfer 598 from a redox tag 540 occurs in an increased amount, or frequency, or rate, when aptamer 570 binding to analyte 19 occurs (e.g., as shown in the figure, when analyte is not bound to aptamer, the redox tag is not available—or is less available—to the electrode, due to, for example, a conformation of the aptamer that hinders or prevents such transfer when not bound to analyte; conversely, when aptamer binds analyte, the conformation of aptamer may change in a manner that positions the redox tag for electron transfer). In various embodiments, aptamer binding to analyte can provide changes in electron transfer and redox current (compared to baseline transfer and current—i.e., transfer/current in the absence of analyte binding) of greater than 5%, greater than 10%, greater than 20%, greater than 50%, greater than 100%, or greater than 200%. For the embodiments illustrated herein, non-limiting examples of electrical measurement techniques may include voltammetry, square wave voltammetry, amperometry, chronoamperometry, coulometry, chronocoulometry, with a preferred embodiment being square wave voltammetry.

FIG. 5B schematically depicts another example of an aptamer 570 with attached redox tag 540 (that differs from the aptamer 570/redox tag 540 schematically shown in FIG. 5A). The embodiment of the aptamer 570 in FIG. 5B is designed such that the redox tag 540 is more available for electron transfer with the electrode 550 in the absence of any analyte 19 binding to the aptamer 570 (high electron transfer—or high ET). Conversely, when analyte 19 binds to the aptamer 570 of FIG. 5B, the redox tag 540 is less available for electron transfer with the electrode 550 (e.g., the redox tag 540 is less exposed—low ET).

FIG. 5C schematically depicts yet another example of an aptamer with attached redox tag 540 (that differs from the aptamer 570/redox tag 540 schematically shown in FIGS. 5A and 5B). The embodiment shown in FIG. 5C is designed with two aptamer portions: a signaling aptamer 572 and an anchor aptamer 574. A redox tag 540 is associated with (such as by being attached to) the signaling aptamer 572. The anchor aptamer 574 includes a portion that has affinity for, and thus can bind, analyte 19. When analyte 19 is not bound to the anchor aptamer 574 (left side of FIG. 5C), the signaling aptamer 572 remains associated with the anchor aptamer 574, and so the redox tag 540 on signaling aptamer 572 is less available for electron transfer with the electrode 550 (low ET). However, once the anchor aptamer 574 binds to analyte 19 (right side of FIG. 5C), signaling aptamer 572 is released from anchor aptamer 574, and the redox tag 540 becomes more available for electron transfer with the electrode 550 (high ET). It will be recognized that the device of the embodiment of FIG. 5C has a plurality of aptamers—and thus includes a plurality of signaling aptamers 572, and a plurality of anchor aptamers 574. As described above, each anchor aptamer of the plurality of anchor aptamers is adapted to bind to analyte. In one embodiment, each signaling aptamer of a majority of the plurality of signaling aptamers is bound to a respective anchor aptamer when a majority of anchor aptamers are not bound to any analyte. (This may occur, for example, prior to the introduction of any analyte.) Once analyte is introduced (such as when a sample fluid is introduced into the device—e.g., by being introduced into the sensor fluid of the device), at least a subset of anchor aptamers from the plurality of anchor aptamers then binds to analyte. When this occurs, a subset of signaling aptamers (from the total plurality of aptamers) dissociates from the anchor aptamers—and the redox tag becomes more available for electron transfer with the electrode.

Further, while the embodiment shown in FIG. 5C depicts analyte 19 binding to anchor aptamer 574 and redox tag 540 on signaling aptamer 572, in an alternate embodiment analyte binding may occur with signaling aptamer (signaling aptamer having redox tag), and binding of analyte to signaling aptamer may serve to release signaling aptamer from anchor aptamer (such as by change in conformation of signaling aptamer). Even further configurations as possible, as understood by those skilled in the art of aptamers. To maximize the signal gain (change in signal) signaling aptamer 572 concentration will typically be less than or equal to the anchor aptamer 574 concentration else the signaling aptamer can cause increased background signal with or without the presence of analyte.

FIG. 5D schematically depicts yet another example of an aptamer with attached redox tag 540 (that differs from the aptamers/redox tags schematically shown in FIGS. 5A, 5B, and 5C). The embodiment of the aptamer 570 in FIG. 5D has both a redox tag 544 and a redox quencher 542 associated therewith (such as by being bound to the aptamer 570). When analyte 19 is not bound to the aptamer 570, the redox tag 544 and redox quencher 542 are spatially separated (left side of FIG. 5D) thereby allowing for greater electron transfer between redox tag 544 and electrode 550 (high ET). However, once the aptamer 570 binds to analyte 19 (right side of FIG. 5D), the redox tag 544 and redox quencher 542 are brought into closer proximity with one another, thereby causing less electron transfer between redox tag 544 and electrode 550 (low ET). Numerous quenchers are possible, including anthraquinone-based redox molecules that can be self-quenching when two of such identical molecules are brought close together (monomer vs. dimer).

FIG. 5E schematically depicts yet another example of an aptamer with attached redox tag 540 (that differs from the aptamers/redox tags schematically shown in FIGS. 5A, 5B, 5C, and 5D). The embodiment of the aptamer 570 in FIG. 5E has both a first redox tag 546 and a second redox tag 548 associated therewith (such as by being bound to the aptamer 570). When analyte 19 is not bound to the aptamer 570, the first and second redox tags 546, 548 are spatially separated (left side of FIG. 5E) thereby allowing for greater electron transfer between first and second redox tags 546, 548 and electrode 550 (high ET). However, once the aptamer 570 binds to analyte 19 (right side of FIG. 5E), the first and second redox tags 546, 548 are brought closer together and the electron transfer from one of the redox tags 546, 548 to the electrode 550 is altered due to a two-step mediated electron transfer process, or other effect, for two redox tags brought into close proximity. These changes in electron transfer are depicted in the voltammograms as shown as 546 a and 548 a. A non-limiting example of redox tags that enable the embodiment of FIG. 5E include methylene blue and ferricyanide.

Turning now to FIGS. 6A-6C, where like numerals refer to like features: FIG. 6A shows a portion of a device 600 that includes a substrate 610, at least first and second electrodes 650, 652, a passivation layer 648 including endogenous solutes 16, a sensor fluid 18 (which, in the embodiment illustrated in FIG. 6A is inside an optional hydrogel 638), a plurality of aptamers 670 having redox tags 640 (free in solution), and a diffusion or iontophoretic pathway 694. Though not part of the device, analytes 19 are also depicted as present in the sensor fluid of the device. FIG. 6A also schematically depicts electron transfer that can occur between redox tags 640 and the first and second electrodes 650, 652. As can be seen in FIG. 6A, as a non-limiting example, electron transfer 698 from the redox tags 640 in an increased amount, or frequency, or rate, when analyte 19 is bound to the aptamer 670. For example, when analyte is bound to aptamer, the hydrodynamic radius or size of the aptamer is smaller and therefore providing a faster diffusion coefficient, which results in the redox tag being more available for electron transfer with the electrodes (such a version will be discussed in greater detail below with respect to FIG. 6B); or, for example, when analyte is not bound to aptamer, the redox tag is not available—or is less available—to the electrode, due to, for example, a conformation of the aptamer that hinders or prevents such transfer when not bound to analyte; conversely, when aptamer binds analyte, the conformation of aptamer may change in a manner that positions the redox tag for electron transfer. In various embodiments, aptamer binding to analyte can provide changes in electron transfer and redox current (compared to baseline transfer and current—i.e., transfer/current in the absence of analyte binding) of greater than 5%, greater than 10%, greater than 20%, greater than 50%, greater than 100%, or greater than 200%. For the embodiments illustrated in FIGS. 6A-6C, non-limiting examples of electrical measurement techniques may include voltammetry, square wave voltammetry, amperometry, chronoamperometry, coulometry, chronocoulometry, with a preferred embodiment being amperometry.

FIG. 6B schematically depicts another example of an aptamer 670 with attached redox tag 640 (that differs from the aptamer 670/redox tag 640 schematically shown in FIG. 6A). The embodiment of the aptamer in FIG. 6B is designed such that the redox tag 640 is less available for electron transfer with the electrodes 650, 652 in the absence of analyte binding to aptamer (left side of FIG. 6B), because of a longer diffusion time between the first and second electrodes 650, 652 where the analyte can undergo redox recycling (e.g. one electrode is a reducing electrode, one electrode is an oxidizing electrode). However, when analyte 19 binds to the aptamer 670 (right side of FIG. 6B), the hydrodynamic radius or size of the aptamer is smaller and therefore providing a faster diffusion coefficient, and therefore redox tag 640 is more available for electron transfer with the first and second electrodes 650, 652. The binding of analyte 19 transforms the aptamer 670 between a long unfolded aptamer 670 (in the absence of analyte 19 binding) and an aptamer 670 with three stems when analyte 19 binds to aptamer 670.

As described above, with respect to FIG. 6A, a non-limiting example of an environment within a device 600 may include an optional hydrogel. In such an embodiment, the hydrogel 638 (such as agar or polyacrylamide) is added to further distinguish diffusion times between aptamers 670 bound to analyte 19 and aptamers 670 not bound to analyte. This is because the hydrogel 638 creates a more tortuous and size-selective diffusion pathway than a pure fluid would by itself. For example, an aptamer 670 that fully dissociates could be modified to have a significant change in hydrodynamic radius (R), which changes its diffusion coefficient (D) according to D=kT/(6πηR). This equation is for diffusion in pure solution; a dense hydrogel 638 can be added to further distinguish the diffusion of the unfolded aptamer vs. the folded aptamer. The resulting current between the redox recycling electrodes is proportional as I∝DC/z, where C is the concentration of the aptamer 670 and z the electrode-to-electrode distance. With respect to changes in signal gain, the diffusion length of oligonucleotides (aptamers) varies with length to the ˜0.6th power, and a 15 kDa protein that is globular/unfolded can have a change in R of 2.15/3.65.

FIG. 6C schematically depicts yet another example of an aptamer 670 with attached redox tag 640 (that differs from the aptamer 670/redox tag 640 schematically shown in FIGS. 6A and 6B). The embodiment of the aptamer in FIG. 6C is designed with two aptamer portions: a signaling aptamer 672 and an anchor aptamer 674. A redox tag 640 is associated with (such as by being attached to) the signaling aptamer 672. The anchor aptamer 674 includes a portion that has affinity for, and thus can bind, analyte 19. When analyte 19 is not bound to the anchor aptamer 674 (left side of FIG. 6C), the signaling aptamer 672 remains associated with the anchor aptamer 674, and so the redox tag 640 on signaling aptamer 672 is less available for electron transfer with the first and second electrode 650, 652 (as the combined signaling and anchor aptamers 672, 674 will exhibit slower diffusion in sensor solution and hydrogel). However, once the anchor aptamer 674 binds to analyte 19 (right side of FIG. 6C), signaling aptamer 672 is released from anchor aptamer 674, and the redox tag 640 becomes more available for electron transfer with the first and second electrodes 650, 652 (as the liberated signaling aptamer 672 will exhibit more rapid diffusion in sensor solution and hydrogel). Further, while the embodiment shown in FIG. 6C depicts analyte 19 binding to anchor aptamer 674 and redox tag 640 on signaling aptamer 672, in an alternate embodiment analyte binding may occur with signaling aptamer (signaling aptamer having redox tag), and binding of analyte to signaling aptamer may serve to release signaling aptamer from anchor aptamer (such as by change in conformation of signaling aptamer).

It will be recognized that when the device shown in FIG. 6A uses the embodiment of aptamers of FIG. 6C, it will include a plurality of signaling aptamers 672, and a plurality of anchor aptamers 674. As described above, each anchor aptamer of the plurality of anchor aptamers is adapted to bind to analyte. In one embodiment, each signaling aptamer of a majority of the plurality of signaling aptamers is bound to a respective anchor aptamer when a majority of anchor aptamers are not bound to any analyte. (This may occur, for example, prior to the introduction of any analyte.) Once analyte is introduced (such as when a sample fluid is introduced into the device—e.g., by being introduced into the sensor fluid of the device), at least a subset of anchor aptamers from the plurality of anchor aptamers then binds to analyte. When this occurs, a subset of signaling aptamers (from the total plurality of aptamers) dissociates from the anchor aptamers—and the redox tag becomes more available for electron transfer with the electrode.

With further reference to FIG. 6C, in addition to changes in diffusion coefficient, the larger the effective sphere for the aptamer the less likely it will experience electron transfer with an electrode (with a first principles estimation based on the inverse of sphere area, proportional to 1/R{circumflex over ( )}2). This example is simply to show that two factors can be at play for embodiments of the present invention, both distance of the redox tag to the electrode and diffusion time to/from the electrode. This diffusion time to an electrode applies other embodiments as well, where for example with a chronoamperometric response for an aptamer the total current baseline could remain higher or reach baseline more quickly as diffusion coefficient for the aptamers increases. This diffusion time to an electrode may also impact interrogation methods such as square wave voltammetry, as aptamer that is near the electrode can contribute to the signal as well if it is able to diffuse to the electrode during each square window (during each voltage pulse that is applied). The first and second electrodes 650 and 652 can be closely spaced via interdigitation or other suitable technique, and, in such an embodiment, may be within less than 50 μm, less than 10 μm, less than 2 μm, or less than 0.4 μm distant of each other.

With reference to FIG. 7 where like numerals refer to like features, another embodiment in accordance with aspects of the present invention is shown. As can be seen in FIG. 7 , a portion of a device 700 is shown, and includes a substrate 710, at least one electrode 750, a passivation layer 748 including endogenous solutes 16, a sensor fluid 18, a plurality of aptamers having redox tags 740 (free in the sensor solution), and a poorly-mobile or non-mobile material 738 in the sensor fluid 18. Though not part of the device, analytes 19 are also depicted as present in the sensor fluid of the device.

The aptamers/redox tags component of the embodiment of FIG. 7 is similar to that shown in FIGS. 5C and 6C, and includes two aptamer portions: a signaling aptamer 772 and an anchor aptamer 774. A redox tag 740 is associated with (such as by being attached to) the signaling aptamer 772. The anchor aptamer 774 includes a portion that has affinity for, and thus can bind, analyte 19. As can be seen in FIG. 7 , the anchor aptamer 774 is immobilized via linkage 739 to the poorly or non-mobile material 738. The poorly-mobile or non-mobile material 738 may comprise various materials, such as a hydrogel. In one non-limiting example, the material 738 could be a hydrogel such as polyacrylamide and the linker be a molecule such as acrydite that is attached to the anchor aptamer at a terminal end or other location. In an alternate embodiment, the anchor aptamer could be cross-linked with other anchor aptamers or the anchor aptamer made so large (e.g., >100 kDa) such that it is effectively immobile in a dense hydrogel 738.

Still referring to FIG. 7 , when analyte 19 is not bound to the anchor aptamer 774, the signaling aptamer 772 remains associated with the anchor aptamer 774, and so the redox tag 740 on signaling aptamer 772 is less available for electron transfer with the electrode 750 (because the combined signaling and anchor aptamers 772, 774 will be poorly-mobile or non-mobile in the sensor fluid due to anchor aptamer 774 being linked to material 738). However, once the anchor aptamer 774 binds to analyte 19, the signaling aptamer 772 is released from anchor aptamer 774 (as indicated by arrow 796), and the redox tag 740 becomes more available for electron transfer with the electrode 750 (because the liberated signaling aptamer 772 will exhibit more rapid diffusion in sensor solution as it is no longer complexed with the anchor aptamer 774 that is linked to poorly-mobile or non-mobile material 738). Further, while the embodiment shown in FIG. 7 depicts analyte 19 binding to anchor aptamer 774 and redox tag 740 on signaling aptamer 772, in an alternate embodiment analyte binding may occur with signaling aptamer (signaling aptamer having redox tag), and binding of analyte to signaling aptamer may serve to release signaling aptamer from anchor aptamer (such as by change in conformation of signaling aptamer).

It will be recognized that the device of the embodiment of FIG. 7 has a plurality of aptamers—and thus includes a plurality of signaling aptamers 772, and a plurality of anchor aptamers 774. As described above, each anchor aptamer of the plurality of anchor aptamers is adapted to bind to analyte. In one embodiment, each signaling aptamer of a majority of the plurality of signaling aptamers is bound to a respective anchor aptamer when a majority of anchor aptamers are not bound to any analyte. (This may occur, for example, prior to the introduction of any analyte.) Once analyte is introduced (such as when a sample fluid is introduced into the device—e.g., by being introduced into the sensor fluid of the device), at least a subset of anchor aptamers from the plurality of anchor aptamers then binds to analyte. When this occurs, a subset of signaling aptamers (from the total plurality of aptamers) dissociates from the anchor aptamers—and the redox tag becomes more available for electron transfer with the electrode.

With reference to FIG. 8 , where like numerals refer to like features, another embodiment in accordance with aspects of the present invention is shown. As can be seen in FIG. 8 , a portion of a device 800 is shown, and includes a substrate 810, at least one electrode 850, a membrane 838, a sensor fluid 18, a plurality of aptamers having redox tags 740 (free in the sensor fluid). Though not part of the device, analytes 19 are also depicted as present in the sensor fluid of the device. The aptamers/redox tags component of the embodiment of FIG. 8 is similar to that shown in FIGS. 5C, 6C, and 7 , and includes two aptamer portions: a signaling aptamer 882 and an anchor aptamer 884. A redox tag 840 is associated with (such as by being attached to) the signaling aptamer 882. The anchor aptamer 884 includes a portion that has affinity for, and thus can bind, analyte 19. The membrane 838 exhibits selective permeability based on size, charge, or at least one solute property, and is able to pass a signaling aptamer 882 but not a signaling aptamer that is attached to a larger anchor aptamer 884. Thus, the membrane 838 impacts the availability of the redox couple 840 to the electrode 850. For example, a signaling aptamer could have a radius of 3 nm/2 nm in folded/unfolded states and an anchor aptamer have 27/7 nm in folded/unfolded state, creating a difference in size of ˜3-10× when a signaling aptamer is freed from an anchor aptamer. Nanofiltration membranes can provide 1s nM pore sizes, and ultrafiltration 10s to 100s nm pore sizes (PES, track-etch, and other materials), resulting in size selective permeability that would enable mainly only the signaling aptamer 882 to penetrate the hydrogel or membrane 838.

And so, still referring to FIG. 8 , when analyte 19 is not bound to the anchor aptamer 884, the signaling aptamer 882 remains associated with the anchor aptamer 884, and so the redox tag 840 on signaling aptamer 882 is less available (or not available) for electron transfer with the electrode 850 (because the signaling aptamer 882 will be unable to cross membrane 838 due to being complexed with anchor aptamer 884). However, once the anchor aptamer 884 binds to analyte 19, the signaling aptamer 882 is released from anchor aptamer 884 and is able to pass through membrane 838, resulting in the redox tag 840 becoming available for electron transfer with the electrode 850. Further, while the embodiment shown in FIG. 8 depicts analyte 19 binding to anchor aptamer 884 and redox tag 840 on signaling aptamer 882, in an alternate embodiment analyte binding may occur with signaling aptamer (signaling aptamer having redox tag), and binding of analyte to signaling aptamer may serve to release signaling aptamer from anchor aptamer (such as by change in conformation of signaling aptamer).

With reference to FIG. 9 , where like numerals refer to like features, another embodiment in accordance with principles of the present invention is shown. In certain of the various embodiments discussed herein, a membrane is used to selectively allow passage of certain molecules and not of others. However, as no membrane is perfectly size selective, and as aptamers and redox tags can degrade over time, it may be advantageous to continually introduce a fresh supply of aptamers, signaling aptamers, and/or anchor aptamers or other solutes that increase performance of the sensor or improve longevity of the sensor (e.g. nuclease inhibitors, for example). Thus, as shown in FIG. 9 , a portion of a device 900 includes substrates 910, at least one electrode 950, a membrane 936, a sample fluid 14, a sensor fluid 18, and a reservoir fluid 17. The membrane 936 exhibits mass flow represented at reference numeral 991, and the device also includes a diffusion restrictive feature 935 (such as a pinhole or membrane) with a mass flow represented at reference numeral 993.

As a nonlimiting example of that shown in FIG. 9 , consider a 0.2 kDa dialysis membrane for membrane 936 and assume the aptamers are 10-100× larger than the solute to be detected (e.g., phenylalanine, cortisol, etc.). Assume the system is designed such that the volume of reservoir fluid 17 is at least one of 2×, 10×, 50×, or 250× greater than volume of sensor fluid 18 and that the mass flow 991 of aptamer is at least 2×, 10×, 50×, or 250× less than mass flow 993 of aptamer, while the mass flow 991 of the analyte is at least 2×, 10×, 50×, or 250× greater than the mass flow 993 of the analyte. As a result, the concentrations of analyte will be within at least 50%, 10%, 2%, or 0.4% of each other when comparing sample fluid 14 with sensor fluid 18, and the concentrations of aptamer will be within at least 50%, 10%, 2%, or 0.4% of each other when comparing sensor fluid 18 and reservoir fluid 17.

As a geometrical example, consider a membrane 936 with 0.2 cm² area and 10% porosity to the analyte, and a diffusion restrictive feature 935 that is a pinhole in materials 910 and 950 0.001 cm² in area and 0.001 cm in length. The mass transport for a small analyte through the membrane will be equivalent to 0.02 cm² area and the mass transport through the feature 935 0.001 cm², which is 20× different, satisfying the above stated criteria for design as shown in FIG. 9 . As a result, both analyte and aptamer concentrations can be maintained for prolonged periods of times (days, weeks, months) even if aptamer is lost from the device or degraded over time. Aptamers could also degrade over time and their presence in the device and the presence of other contaminants such as nucleases or proteins could be problematic. For example, if signaling aptamers became cleaved and their molecular weight decreased, they could give a false higher reading of signal in embodiments of the present invention. With membrane protection of the sensor fluid from the sample fluid, most degradation or contamination modes will be very slow, such that the reservoir may also act as a waste removal element.

With further reference to FIG. 9 , various aspects of the present invention are taught in greater detail with respect to isolation of aptamer (or the plurality of aptamers) in the sensor fluid 18 from the sample fluid 14 and reservoir fluid 17. The principles described with respect to FIG. 9 apply broadly to other embodiments of devices disclosed herein. In that regard, consider an aptamer sensor for creatinine or phenylalanine, (which have molecular weight of only 131 Da and 165 Da, respectively), and an aptamer having a molecular weight of 10-15 kDa (which is common for many aptamers). In such a situation, a membrane 936 having a 150 Da molecular weight cutoff could be used to prevent movement of aptamers from sensor fluid to another fluid (like sensor fluid to sample fluid). Alternatively, commercial membranes such as Dow FilmTech Polyamide membranes with 200-400 Da cutoffs may be suitable for use as membrane 936. Further still, non-limiting examples of alternate materials for the membrane 936 include cellulose acetate, polypiperazine-amide, and polydimethylsiloxane. But even in such a case, conventional membranes are designed for pressure-driven separations, and so include a significant thickness or a backing layer, which increases overall thickness (on the order of 100s of μM). A thick membrane (100's to 1000's of μm), such as these would impart a penalty on device response time to changes in analyte concentration, as the analyte must diffuse through the thick membrane (a thicker membrane 36 not only creates a more tortuous path for analyte diffusion, but it increases the distances over which the analyte concentration gradient exists between sample and sensor which further decreases the diffusive flux). Therefore, in one embodiment of the present invention the membrane backing material can be facing the sample fluid 14 to resolve this lag time increase at least in part because backing material typically has high porosity.

As an additional example, consider phenylalanine and a ˜90 nm thick epoxy membrane 936 with an effective diffusion coefficient divided by membrane 936 thickness of Deff/Δx of between about 5 m s⁻¹×10⁻³ and 0.005 m s⁻¹×10⁻³, as taught by Rodler et al. in Freestanding ultrathin films for separation of small molecules in an aqueous environment, Journal of Biotechnology, Volume 288, Dec. 20, 2018, pages 48-54. (https://doi.org/10.1016/j.jbiotec.2018.10.002). The thickness or porosity of the membrane 936 can be adjusted easily. Accordingly, the present invention may benefit from a membrane 936 that has a Deff/Ax of, in one embodiment, at least 5 m s⁻¹×10⁻³. In another embodiment, the Deff/Ax is at least 0.5 m s⁻¹×10⁻³. In yet another embodiment, the Deff/Ax is at least 0.05 m s⁻¹×10⁻³. In another embodiment, the Deff/Ax is at least 0.005 m s⁻¹×10⁻³. Membranes 936 of the present invention are, in one embodiment, less than about 100 nm thick. In another embodiment, membranes of the present invention are less than 1 μm thick. In yet another embodiment, membranes of the present invention are less than 10 μm thick. In another embodiment, membranes of the present invention are less than 100 μm thick. A membrane 936 with a well-designed Deff/Δx of at least 5 m s⁻¹×10⁻³ and thickness of sensor fluid 18 of ˜1-10 μm, can enable a device on/off time for the sensor to measure 90% of the sample fluid concentration in at least <15 min.

Additionally, in some cases, the analyte molecular weight will become larger or be too large to permeate the membrane 936, and the aptamer might permeate the membrane 936 due to its molecular weight or due to a stranded geometry that allows it to navigate through a membrane similar to a rope being pulled or pushed through a screen (if the rope were balled up, it could not be pushed through the screen). Therefore, alternate methods of isolating the aptamer from sample fluid 14 are needed in some cases. One important factor is the retention % vs. molecular weight, (see the graph for a membrane 936 as illustrated in FIG. 4 ). This is also referred to as retentivity. A typical membrane 936 can provide >90% retention for example at 10 kDa, and <9% retention at <1 kDa for a change in retentivity/change in molecular weight of ˜1×. This is not highly selective with respect to the present invention, because for example, with an implanted device, aptamers could be slowly and continually lost over time. Therefore, the present invention may benefit from a membrane with a change in retentivity/change in molecular weight that, in one embodiment, is at least 2×. In another embodiment, the membrane has a change in retentivity/change in molecular weight that is at least 5×. In yet another embodiment, the membrane has a change in retentivity/change in molecular weight that is at least 10×. In one embodiment, the membrane has a change in retentivity/change in molecular weight that is at least 20×.

Further, as taught in other embodiments, while some aptamer may be continually lost from the sensor fluid, fresh aptamer can diffuse in from an adjacent reservoir to replenish lost aptamer. This reservoir is shown in FIG. 9 as including reservoir fluid 17. As an example, the volume of sensor fluid 18 could be 1 μL in volume using an area of membrane 936 of 0.1 cm² and a separation distance between membrane 936 and electrode 950 of 0.01 cm (100 μm). A reservoir, including reservoir fluid 17, which in turn includes fresh aptamer solution could be in fluid communication with the volume including sensor fluid 18 via a pore 995 that is only 0.001 cm², such that analyte and aptamer would very slowly diffuse into/out of the aptamer reservoir, again, shown in FIG. 9 as including reservoir fluid 17, but such that new aptamer would constantly diffuse into the volume including the sensor fluid 18 as any aptamer is lost from the sensor fluid 18 through the membrane 936 to the sample fluid 14. In one embodiment, the pore 995 is a fluidic connection from the reservoir, including the reservoir fluid 17, and the volume, including the sensor fluid 18.

Such an approach could allow for protein sensing. For example, assume a sensor fluid 18 volume of 100 nL, and a membrane 936 that retains 90% of the aptamer over 6 hours, and which can allow a protein such as luteinizing hormone to diffuse into the sensor fluid 18 and achieve 90% of sample fluid 14 concentration of the hormone within 12 hrs. This would allow a device 900 to sufficiently measure luteinizing hormone for fertility monitoring applications. Now, if the reservoir including reservoir fluid 17 with aptamer had a volume of 200 μL, then it could lose 10% of its aptamer before a sensor signal would be impacted by 10%. If the device 900 is losing aptamer through the membrane 936 at a rate of, for example, 10% of aptamer every 6 hours in the 100 nL volume including sensor fluid 18, then with the 200 μL reservoir including reservoir fluid 17, the device 900 could last 2000× longer or 12,000 hours or >16 months, more than long enough for creating an implantable device 900. Therefore, depending on the volume of the reservoir including reservoir fluid 17 and scaling of other device 900 dimensions and membrane 936 porosity, the present invention can retain 90% of the initial aptamer concentration in the sensor fluid 18 for at least >16 months, >8 months, >4 months, >2 months, >1 month, >2 weeks or >1 week.

Further, and as will now be described in greater detail, there is no major penalty if the aptamer is designed such that one end of the aptamer is inactive and increases the total molecular weight of the aptamer by at least 50%. For example, in some embodiments, the aptamer includes an active end configured to bind to the analyte and which has the redox tag. In some embodiments, the aptamer may include a longer inactive end configured to provide molecular weight or size to the aptamer and/or configured to reduce aptamer permeation through the membrane 936. The longer inactive end may be configured to be rigid or have at least one permanent fold, wherein the rigid aptamer or aptamer including a permanent fold is dimensionally larger than a non-rigid aptamer or aptamer not including a permanent fold. In one embodiment, the molecular weight of such aptamers is at least >15 kDa. In another embodiment, the molecular weight of such aptamers is at least >30 kDa. In yet another embodiment, the molecular weight of such aptamers is at least >60 kDa. In one embodiment, the molecular weight of such aptamers is at least >120 kDa. For example, that active end aptamer could have a molecular weight of at least <20, <10, or <5 kDa, and the inactive end of the aptamer may be configured to be folded and therefore configured to increase the total size and molecular weight of the aptamer.

In additional embodiments, aptamers may be attached to other materials, or to nanoparticles, to also help isolate them from the sample fluid. For example, an aptamer could be attached to a polyethylene glycol polymer, the polyethylene glycol polymer may have a molecular weight of about 300 kDa, which can be referred to as a ‘particle’. Particles could be other polymers, metal such as gold, carbon, or iron-oxide and can be, in different embodiments, at least >1 nm, >3 nm, >10 nm, >30 nm, or >100 nm in diameter and still stably dispersed in solution as is known using one or more methods like those used in the art of pigment and nanodispersions. Aptamers can be bound to iron nanoparticles using, as a non-limiting example, dibromomaleimide (DBM)-termination, and bound to gold nanoparticles using thiol termination. With use of magnetic nanoparticles such as iron-oxide, the aptamer isolation element may also be a magnet that retains the nanoparticles near the membrane with or without use of a membrane, and in this example the aptamer isolation element is a magnet. This approach could allow the present invention to measure a protein analyte, for example a 30 kDa protein with an average diameter of <5 nm.

As described previously, the aptamers may be optically tagged aptamers having a fluorescent tag. In such embodiments, a fluorescence quencher may be used as well. For example, with reference to FIG. 1 , layer 120 could be a semi-transparent light source such as organic light emitting diode and electrode 150 could be a light detector such as an inorganic photodiode facing the sample fluid with or without a light filter to increase signal to noise ratio. Aptamer could be tagged with a fluorescent optical tag that is excited by the light source 120 and then emits light that is detected by the electrode 150. Although not shown, the present invention may also include a fluorescence quencher that is tagged to the aptamer such as a black-hole quencher. In some embodiments, the analyte itself is the fluorescence quencher (e.g. NADH, FAD, elastin, collagen, tryptophan, keratin) by absorbing the energy from the fluorescence tag and emitting the light at a wavelength different than the fluorescence tag.

As anon-limiting example of the above, a typical optical aptamer probe is 25 nucleotides long. 15 of the nucleotides are complementary to or have a strong binding affinity for the analyte target and do not base pair with one another, while the five nucleotides at each terminus are complementary to each other rather than having strong affinity to the target analyte. A typical molecular beacon structure can be divided in 4 parts: 1) loop, an 18-30 base pair region that is complementary to or has strong binding affinity for the target analyte; 2) stem formed by the attachment to both termini of the loop of two short (5 to 7 nucleotide residues) oligonucleotides that are complementary to each other; 3) 5′ fluorophore at the 5′ end of the aptamer, a fluorescent dye is covalently attached; 4) 3′ quencher (non-fluorescent) dye that is covalently attached to the 3′ end of the aptamer. When the aptamer is in closed loop shape, the quencher resides in proximity to the fluorophore, which results in quenching the fluorescent emission of the latter. When the aptamer is open (e.g. caused by analyte binding) the fluorescence is not quenched. A closed loop structure is not required for the present invention, as the fluorescence tag and quencher can experience an average change in their distance with analyte binding, similar to that taught in previous embodiments of the present invention where a redox tag experiences an average change in distance to an electrode surface.

Because passivating layers can degrade over time for blocking materials such as mercaptohexanol, in one embodiment the sensor fluid 18 and/or reservoir fluid 17 contains a passivating solute such as peptides (e.g., albumin) that can continually re-attached to the electrode and support a long-lasting and predictable passivating layer. Although albumin may exist in the sample fluid 14, it may be unable to reach the sensor fluid 18 due to the membrane 936.

Although not illustrated in detail herein, the embodiments of the present invention may also be applied to a simple single-use device. For example, device 100 of FIG. 1 could be applied to the skin 12 and take a single measurement of an analyte within 15 minutes of application of the device 100 and then removed from the skin 12 surface. Alternately (not shown) a device format similar to a glucose test-strip or other device with a microfluidic channel or a wicking channel (like a lateral flow assay) can be used to transport a sample fluid to one or more sensors and embodiments as taught herein for the present invention. For example, the aptamers and redox tags could be dry and placed near the inlet of such a test strip and dissolved into the sample fluid and then transported to a membrane-protected working electrode where measurement of the analyte is performed.

Examples Example 1

With reference to FIGS. 10A and 10B a cortisol binding aptamer was utilized in a manner similar to that taught in FIG. 6C, where the signaling aptamer 672 was tagged with methylene blue as a redox tag 640 with an aptamer sequence of GTCGTCCCGAGAG [SEQ ID NO. 1] and where the anchor aptamer 674 with a sequence of ctctcgggacgacGCCCGCATGTTCCATGGATAGTCTTGACTAgtcgtccc [SEQ ID NO. 2].

Electrodes 650, 652 were gold interdigitated electrodes with a 5 μm spacing in between them. The gold electrodes were passivated with an exogenous molecule of mercaptohexanol. No hydrogel 638 was utilized in this experiment. The sensor solution was buffer solution with 5 μM of the aptamers 650, 652 in solution, and a reference electrode of platinum was used. The device 600 was measured amperometrically vs. a titration curve of cortisol as the analyte 19. The results are shown in FIG. 10A and FIG. 10B (open circles, open diamonds, and solid diamonds), and a control experiment with titration of simply adding more cortisol but without aptamer in solution is also shown in FIG. 10B (solid circles). The signal gain in Example 1 is as much as 70%, and if the anchor aptamer was made even larger or smaller the signal gain could be tuned to be as much as 200% or more as little as 5% based on the change in diffusion rate of the signaling aptamer to the electrode compared to the signaling aptamer when it is bound to the anchor aptamer. Signal gain is also measured above a baseline signal, and changing signaling aptamer concentration can therefore be used to tune the signal gain.

Example 2

The experiment of Example 1 was repeated but instead of using mercaptohexanol passivation of the gold electrodes 650, 652, endogenous small molecule solutes found in blood or interstitial fluid were allowed to passivate the gold electrode 650, 652. It was found that without passivation background current was very high, but that both mercaptohexanol and endogeneous solutes were able to adequately reduce background current and enable sensor operation.

Example 3

Sensors were tested with scanning wave voltammetry, and redox peaks via voltammetry were observable with 100 nM of aptamer. Higher aptamer concentrations only increase the amount of signal and 1 μM, and 10 μM and 100 μM of aptamer were tested as well. Generally, lower aptamer concentrations were preferred as they reduce device lag times as they require less concentration of analyte to create a change in sensor signal.

Example 4

Consider an example as shown in FIG. 6C that teaches the impact of device parameters on device lag time. Assume a device that operates with a sample fluid that is interstitial fluid, where redox active interferents such as NADH/NAD+ (663 Da) are ˜20 nM each in serum (so 50 nM total). Diffusion coefficient is roughly proportional to the cube root of molecular weight. Accordingly, a 5 kDa aptamer would have diffusion coefficient that is 3× greater than NADH, and with the Cotrell equation, the current difference between NADH and the signaling aptamer would be 3{circumflex over ( )}(½) or 1.73×. Therefore to have 10× more current from signaling aptamer than NADH/NAD+ would require 50 nM*10*1.73=865 nM of aptamer. Next, round up and assume ˜1 μM of signaling aptamer to provide a safe margin on aptamer signal strength vs. interferents. If the distance between the membrane and electrode was 5 μm, then for cortisol at 10 nM and 1 μM aptamer the ‘equivalent’ volume of sensor fluid is 100× greater or 500 μm thick from a lag time perspective. Next, assume a membrane that is 10% porous to cortisol, for this configuration, the cortisol can diffuse into the sensor fluid and reach 90% of its concentration in the sample fluid in less than 20 minutes. Next, utilize a membrane that is 1.66% or 50% porous to cortisol, and the lag time becomes 60 min or as little as 4 min, respectively. Aptamer concentration can be increased or decreased to adjust this lag time, the distance between the membrane and electrode and/or substrate can be modified to adjust this lag time by adjusting the sensor fluid volume, and for different analytes a higher or lower analyte concentration will also adjust this lag time (e.g. cortisol at 1 nM will have 10× greater lag time while cortisol at 100 nM would have 10× lesser lag time. Therefore generally, the present invention can enable devices with lag times to reach 90% of sensor response that are less than at least one of 180 min, 60 min, 20 min, 5 min, 2 min.

Example 5

Example 4 illustrates that a smaller sensor fluid volume is advantageous to achieve short lag times. A smaller sensor fluid volume may also allow one to keep the lag time constant but to reduce the sensor fluid volume down in a matter that allows the membrane porosity to decrease proportionally as well such that device performance such as longevity (aptamer isolation from sample fluid) is improved. Small volumes for the sensor fluids can be achieved in one or more ways and applied to planar or wire electrode formats. For example, a gold electrode can be coated with a sugar solution (sucrose or trehalose) or a water soluble polymer such as poly-vinyl-alcohol containing the aptamers and dried to a solid film coating on the gold electrode. Next, the membrane can be solution case using a solvent or fluid system that does not dissolve the solid-coated layer containing aptamer (for example alkanes to dissolve acrylic or styrene, or flourosolvents to dissolve Teflon, where they are cast and rapidly dried to create porosity). The cast membranes can also contain dispersions of water soluble polymer or water soluble materials, such that when the membrane is coated and dried, when the device is wetted with water or sample fluid all the water soluble materials dissolve away, including the sugar used to coat the aptamers. Generally, the present invention may use orthogonal solvent systems (e.g., water and oil) to allow layer by layer fabrication of as solid aptamer layer and membrane that become filled with solution during use of the device with a sample fluid. Alternately, the membrane can be deposited from a gas such as spray coating of nano beads or electrospinning, and the solid layer containing aptamer again is not harmed by fabrication of the membrane. The space between the membrane and the electrode or the substrate the electrode is fabricated on can also be regulated by one or more spacer materials, such as monodisperse spacer balls that are coated along with the aptamer and sugar layer. For example, silica nanoparticles can be purchased in sizes ranging from 20 nm to 1 μm, and microbeads up to 10 or even 100 μM. Therefore, the present invention includes at least one method to fabricate a solid layer of aptamer that becomes a fluid during use and onto that solid layer of aptamer the membrane is coated. Therefore, the present invention enables a spacing between the electrode and the membrane that is at least one of less than 100 μm, 10 μm, 1 μm, 0.1 μm, or 0.01 μm.

Example 6

With respect to an embodiment of the present invention, consider a device where lag-time and/or sample volume is a concern, because a high concentration of aptamer in the sensor solution can require a high amount of analyte that must diffuse from the sample fluid into the sensor fluid and the limiting factor for lifetime is fatigue of the redox tag (limited reversibility over time). An embodiment of the present invention includes a sensor where the working electrode area is equal to the membrane area and as taught in other embodiments, the thickness of the sensor fluid is 5 μm. In one embodiment, the membrane has an area of 0.2×0.2 cm or 0.04 cm² and the working electrode an area of 0.02×0.02 cm or 0.0004 cm², the lifetime of the device therefore improves 100× further while not sacrificing lag-time because less aptamer and redox couple would be interrogated due to the smaller working electrode area. In alternate embodiments, the working electrode area and membrane area have a ratio of <10, <1, <0.1, <0.01 or <0.001. This same approach can be used to reduce the sample volume by allowing the volume of the sensor fluid to be reduced further as the working electrode is further reduced.

Although not described in detail herein, other steps which are readily interpreted from or incorporated along with the disclosed embodiments shall be included as part of the invention. The embodiments that have been described herein provide specific examples to portray inventive elements, but will not necessarily cover all possible embodiments commonly known to those skilled in the art. 

What is claimed is:
 1. A sensing device for sensing at least one analyte comprising: a sensor fluid including a plurality of aptamers, one or more aptamers of the plurality of aptamers being configured to sense an analyte included in a sample fluid, the plurality of aptamers configured to freely diffuse in the sensor fluid; and at least one isolation element retaining the plurality of aptamers in the sensor fluid.
 2. The device of claim 1, wherein the isolation element is a membrane.
 3. The device of claim 1, wherein the one or more aptamers of the plurality of aptamers are configured to measure the analyte electrochemically and the one or more aptamers each includes at least one redox tag and the device includes at least one electrode.
 4. The device of claim 1, wherein the one or more aptamers of the plurality of aptamers are configured to measure the analyte optically and the one or more aptamers includes at least one fluorescent tag, at least one optical source, and at least one optical detector.
 5. The device of claim 1, further comprising a reservoir fluid that is in fluidic communication with the sensor fluid.
 6. The device of claim 5, wherein a volume of reservoir fluid is at least one of 2×, 10×, 50×, 250× greater than a volume of the sensor fluid.
 7. The device of claim 5, wherein a first mass flow of aptamer through the isolation element and a second mass flow of aptamer through a fluidic connection between the sensor fluid and the reservoir fluid, and the first mass flow is at least 2×, 10×, 50×, 250× less than the second mass flow.
 8. The device of claim 5, wherein there is a first mass flow of analyte through the isolation element and a second mass flow of analyte through a fluidic connection between the sensor fluid and the reservoir fluid, and the first mass flow is at least 2×, 10×, 50×, 250× greater than the second mass flow.
 9. The device of claim 7, wherein a concentration of aptamer in the sensor fluid is within at least 50%, 10%, 2%, 0.4% of the concentration in the reservoir fluid.
 10. The device of claim 8, where in the concentration of analyte in the sensor fluid is within at least 50%, 10%, 2%, 0.4% of the concentration in the sample fluid.
 11. The device of claim 2, wherein the membrane includes a backing material and the backing material is facing the sample fluid.
 12. The device of claim 2, wherein the membrane has a Deff/Δx that is greater than an amount selected from the group consisting of 5 m s⁻¹×10⁻³, 0.5 m s⁻¹×10⁻³, 0.05 m s⁻¹×10⁻³, and 0.005 m s⁻¹×10⁻³.
 13. The device of claim 2, wherein the membrane has a thickness that is less than an amount selected from the group consisting of ˜100 nm, 1 μm, 10 μm and 100 μm thick.
 14. The device of claim 2, wherein the membrane has a change in retentivity/change in molecular weight that is greater than an amount selected from the group consisting of 2×, 5×, 10×, and 20×.
 15. The device of claim 5, wherein the device is configured to retain 90% of the initial aptamer concentration in the sensor fluid for a period of time selected from the group consisting of >16 months, >8 months, >4 months, >2 months, >1 month, >2 weeks, and >1 week.
 16. The device of claim 1, wherein each aptamer of the plurality of aptamers includes at least an active portion and an inactive portion, the inactive portion comprising at least 50% of the total aptamer molecular weight.
 17. The device of claim 16, wherein the inactive portion of the aptamer is rigid by having a secondary structure wherein the aptamer is bound to itself.
 18. The device of claim 16, wherein the inactive portion of the aptamer includes at least one permanent fold.
 19. The device of claim 16, wherein the aptamer has a molecular weight that is an amount selected from the group consisting of >15 kDa, >30 kDa, or >60 kDa and >120 kDa.
 20. The device of claim 16, wherein the active portion of the aptamer has a molecular weight that is an amount selected from the group consisting of <20, <10, and <5 kDa.
 21. The device of claim 1, wherein a majority of the plurality of aptamers are bound to a plurality of nanoparticles.
 22. The device of claim 21, wherein the nanoparticles are magnetic and the isolation element is a magnet.
 23. The device of claim 21, wherein the size of the particles is selected from the group consisting of >1 nm, >3 nm, >10 nm, >30 nm, and >100 nm in diameter.
 24. The device of claim 1, wherein the analyte is a small molecule of molecular weight less than 1000 Da.
 25. The device of claim 1, wherein the analyte is a protein.
 25. The device of claim 1, wherein the sensor fluid includes at least one passivating solute.
 26. The device of claim 1, wherein the device is a continuous sensing device.
 27. The device of claim 1, wherein the device is a single-use device.
 28. The device of claim 1, wherein the device has a lag time, and the lag time to reach 90% of a sensor response is less than at least one of 180 min, 60 min, 20 min, 5 min, 2 min.
 29. The device of claim 1, wherein the device further comprises a surface opposite the isolation element, the surface defining a volume housing the sensor fluid and a distance between the surface and the element is at least one of less than 100 μm, 10 μm, 1 μm, 0.1 μm, 0.01 μm.
 30. The device of claim 3, wherein the electrode has an area and the membrane has an area, and the electrode area and membrane area have a ratio of <1, <0.1, <0.01 or <0.001.
 31. The device of claim 3, wherein a passivating layer is present on the electrode.
 32. The device of claim 3, wherein the redox tag is configured to move within the sensor fluid, and the movement of the redox tag results is a change of electron transfer between the redox tag and the electrode.
 33. The device of claim 1, wherein the plurality of aptamers comprise a plurality of signaling aptamers and a plurality of anchor aptamers.
 34. The device of claim 33, wherein the plurality of redox tags are bound to the signaling aptamers, but are not bound to the anchor aptamers.
 35. The device of claim 1, wherein the aptamer is a folded aptamer.
 36. The device of claim 1, wherein the aptamer is an unfolded aptamer.
 37. The device of claim 2, wherein the membrane has a Deff/Δx of between about 5 m s⁻¹×10⁻³ and 0.005 m s⁻¹×10⁻³.
 38. The device of claim 2, wherein the membrane has a Deff/Δx that is at least 0.005 m s⁻¹×10⁻³.
 39. The device of claim 2, wherein the device is configured to measure 90% of the sample fluid concentration in at least <15 min.
 40. The device of claim 1, wherein one or more aptamers of the plurality of aptamers is tagged with a fluorescent optical tag.
 41. The device of claim 41, further comprising a fluorescence quencher.
 42. A method of fabricating a sensing device for at least one analyte comprising: defining a first volume configured to house a sample fluid with at least one analyte; defining a second volume configured to house a sensor fluid, the sensor fluid including an analyte that is at least in part measured by an aptamer freely diffusing in the sensor fluid; retaining the aptamer in the sensor fluid with at least one isolation element.
 43. The method of claim 42, wherein the aptamer is coated as a solid layer and the isolation element is coated onto a solid layer including the aptamer.
 44. The method of claim 43, wherein spacer material is coated along with the aptamer in the solid layer.
 45. The method of claim 42, further comprising fabricating a solid aptamer layer and the isolation element with solvent systems that are orthogonal; and fabricating the isolation element configured to be filled with a solution during use of the device with a sample fluid.
 46. The method of claim 42, further comprising defining a reservoir configured to house a reservoir fluid, wherein the sensor fluid is in fluid communication with the reservoir.
 47. The method of claim 42, wherein the sensor fluid is in fluid communication with the reservoir via a pore.
 48. The method of claim 42, wherein the isolation element is a membrane.
 49. The method of claim 42 further comprising housing an electrode in the second volume.
 50. The method of claim 49 further comprising spacing the electrode and the isolation element at least one of less than 100 μm, 10 μm, 1 μm, 0.1 μm, or 0.01 μm apart.
 51. A method of sensing an analyte in a sample solution, the method comprising: bringing an analyte included in a sample fluid into contact with an aptamer included in a sensor fluid, the aptamer being freely diffusing in the sensor fluid, the contact of the aptamer with the analyte resulting in a change in the electron transfer between a redox tag and an electrode; and measuring the change in electron transfer between the redox tag and the electrode.
 52. The method of claim 51, wherein the sample fluid is in fluid communication with the sensor fluid, and wherein the sensor fluid is in fluid communication with a reservoir fluid, the method further comprising: flowing the aptamer from the sensor fluid to the sample fluid at a first rate; and flowing the aptamer from the reservoir fluid to the sensor fluid at a second flow rate.
 53. The method of claim 52, wherein the first rate is at least 2×, 10×, 50×, or 250× less than the second rate.
 54. The method of claim 51, wherein the sample fluid is in fluid communication with the sensor fluid, and wherein the sensor fluid is in fluid communication with a reservoir fluid, the method further comprising: flowing the analyte from the sensor fluid to the sample fluid at a first rate; and flowing the analyte from the sensor fluid to the reservoir fluid at a second flow rate.
 55. The method of claim 54, wherein the first rate is at least at least 2×, 10×, 50×, or 250× greater than the second rate.
 56. The method of claim 51, wherein the analyte is a protein.
 57. The method of claim 51, wherein measuring a change in electron transfer between the redox tag and the electrode has a lag time, and the lag time to reach 90% of a sensor response is less than at least one of 180 minutes, 60 minutes, 20 minutes, 5 minutes, and 2 minutes.
 58. The method of claim 51, wherein the redox tag is attached to the aptamer. 